You need to identify whether the agency has a full-fledged UX team capable of carrying out and implementing User Research, Information Architecture, Interaction Design, Prototyping, Usability, Visual Design, and Front-end Development. More specifically, there needs to be a clear divide between user experience, user interface design, and development. If they have one person who does all three or even two of these functions, then you’re definitely not dealing with a specialized UX design agency. Also pay attention to the ratio of developers vs. designers. A design-oriented agency will usually have more designers than developers, while a development-oriented agency will have more software engineers on the team.